You need enough points to book at least 2 nights in order to walk a reservation. Having enough free points for just one night won't cut it.
My advice would have been to book the BLT GV for the first night right now with the BLT points you have and then modify it to add the nights with your OKW points.
But you're now inside your 7 month window for March 10 (which would have been the first day of a walk). Modify your OKW reservation to go from 3/10-3/17 if you can. Book the night of 3/17-3/18 with your BLT points. That gives you 8 nights (one night short) but it starts a day too early.
Tomorrow, if 3/17-18 is still open, modify the first reservation to 3/11-3/18. Modify the 3/17-3/18 reservation, which used BLT points, to 3/18-3/19. You now have 8 nights but at least they start on the correct day.
Repeat the next day by modifying the 3/11-3/18 reservation to add another night (if available) so that it becomes 3/11-3/19. Modify the 3/18-3/19 to 3/19-3/20. You now have all of your nights in 2 reservations.
Then, have MS either merge or link the reservations so that you don't need to move.
The risk is that it's possible that there is only one GV in inventory and when you grab it with your BLT points, none will be available for you to walk the reservation made with the OKW points points.